Potassium binding sites in muscle: electron microscopic visualization of K, Rb, and Cs in freeze-dried preparations and autoradiography at liquid nitrogen temperature using 86Rb and 134Cs.

Normal frog sartorius muscles and muscles in which a major portion of the intracellular K+ was reversibly replaced by Rb+ or Cs+ were frozen, freeze-dried and embedded without chemical fixation or staining. Dry-cut sections of these preparations reveal striation patterns with higher contrast than those of wet-cut sections of the same preparation. The results suggest that in the living state the alkali metal ions are mainly localized in the A bands and Z lines of myofibrils. This idea is confirmed by a new autoradiographic technique by means of which the distribution of Rb+ and Cs+ in frozen-hydrated single muscle fibers has been investigated. The findings support the association-induction hypothesis according to which most cell K+ and other alkali-metal ions are not free in cell water but are adsorbed to beta- and gamma-carboxyl groups of cell proteins.